1. Field of the Invention
The present invention relates to a data conversion method for converting digital data into signals suitable for a recording or transmission system used for recording or transmitting the digital data, and also relates to a magnetic recording and reproduction apparatus employing the same data conversion method.
2. Description of the Related Art
Generally, when transmitting data through a transmission channel, it is required to use the transmission channel effectively and transmit data with minimum data degradation during transmission. The achieve this, various data conversion methods have been proposed, depending on the kind of the information source and the characteristics of the transmission channel. On the other hand, the characteristics required of the data conversion method vary according to the characteristics of the transmission channel. One example is a transmission channel system having a band-pass characteristic, one such system being a magnetic recording and reproduction system.
Specifically, in a magnetic recording and reproduction apparatus such as a rotary head type VTR, since signals for recording or reproduction are passed through a rotary transformer, head amplifier capacitors, etc., the DC component and low-frequency components are cut off. If the DC component and low-frequency components cannot be recorded or reproduced, waveform distortion and fluctuations in the DC level will be caused in the reproduced digital signal, which tends to result in digital signal errors. To avoid this, data conversion is performed on the digital signal to be recorded so that the digital signal, when recorded or reproduced, contains as little DC component and low-frequency components as possible. Such a data conversion method is generally known as a DC-free data conversion method. Examples of the DC-free conversion method include an 8-10 modulat on method, such as described in Japanese Patent Application Laid-Open Nos. 56-19506(1981) and 60-48646(1985), "THE DAT CONFERENCE STANDARD" (issued in June 1987), etc., and an 8-14 modulation method, such as disclosed in Japanese Patent Application Laid-Open Nos. 61-30818 (1986) and 3234146(1991), etc.
The 8-10 modulation method is a data conversion method for converting 8-bit digital data (dataword) into 10-bit digital data (codeword), and the 8-14 modulation method is a data conversion method for converting an 8-bit dataword into a 14-bit codeword. These DC-free data conversion methods convert data so that the DSV (digital sum variation) becomes zero.
The DSV represents the value of the integral of binary levels of 1 (high level) and 0 (low level) which are represented by +1 (positive) and -1 (negative), respectively. The DSV has a value taken over a given period at a given time. When the DSV is obtained for a continuing binary signal, if the DSV increases or decreases infinitely, the binary signal has a DC component, and if the DSV is finite, then the binary signal does not have a DC component. A CDS (codeword digital sum) represents a DSV from the beginning to the end of a codeword.
There are three kinds of codewords: a codeword with CDS&lt;0, a codeword with CDS=0, and a codeword with CDS&lt;0. For example, in the data conversion method disclosed in Japanese Patent Application Laid-Open No. 59-123343(1984), codewords with CDS=0 are mapped in one-to-one corresponding relationship to datawords, while codewords with CDS&gt;0 and CDS&lt;0 are paired together and each pair is mapped in corresponding relationship to one dataword. By monitoring the DSV, a codeword with CDS.ltoreq.0 is selected when DSV&gt;0, and a codeword with CDS.gtoreq.0 is selected when DSV&lt;0. When DSV=0, an appropriate codeword is selected.
For magnetic recording and reproduction apparatus such as rotary head type VTRs, it is desirable that Tw.times.Tmin be large and yet high-density recording be made possible (Tw is the detection window width that represents the margin against symbol errors when the time base for the reproduced signal becomes unstable due to jitter, and Tmin is the minimum spacing between magnetic reversals that corresponds to the resolution of the recording system). Furthermore, it is desirable that the peak shift due to intersymbol interference be small and also that Tmax/Tmin (where Tmax is the maximum spacing between magnetic reversals) be small for improved overwrite characteristics due to signal overwriting. It is also desired that Tmin be large in order to minimize the high-frequency components. FIG. 1 shows the values of Tmin, Tmax, Tmax/Tmin, Tw, Tw.times.Tmin, and DSVmax in 8-10 modulation and 8-14 modulation.
FIG. 2 is a diagram showing the configuration of a modulation circuit employing the 8-10 modulation method, FIG. 3 is a diagram for explaining the operation of the same, and FIG. 4 is a diagram showing a part of a data conversion table. In FIG. 2, the reference numeral 70 designates an encoder which accepts 8-bit digital data and a 1-bit table selection signal (Q') at its inputs and which bit signal (Q) for selecting the table for the next codeword; and 71 indicates a flip-flop for delaying the codeword table selection signal by one dataword. The encoder 70 includes a read-only memory (ROM) or the like that holds the contents of the data conversion table shown in FIG. 4; in the table, codewords with CDS=0 are mapped in one-to-one corresponding relationship to 256 datawords from "00" to "FF" hex, while in the case of codewords of CDS.noteq.0, pairs of codewords, one with CDS=+2 and the other with CDS=-2, are each mapped in corresponding relationship to one dataword, the table for Q'=-1 consisting of codewords with CDS =+2 and the table for Q'=+1 consisting of codewords with CDS=-2. The table selection signal Q is used to select the CDS (the table) of the direction that suppresses the divergence of charges in the code sequence.
Next, the operation of the above circuit will be described with reference to FIGS. 2 and 3. As shown in FIG. 3, an 8-bit dataword "FF" first input to the encoder 70 has a table selection signal Q'=-1, and consequently, the encoder 70 outputs a 10-bit codeword "1111101010" with CDS=+2 corresponding to "FF" for Q'=-1. At the same time, the table selection signal Q=-1 is outputted for table selection for the next codeword. This 10-bit parallel signal is then converted to a serial signal with MSB as the leading bit, after which the signal is NRZI-modulated. As a result, the DSV value at the end of the codeword becomes +2.
Next, when "00" is inputted to the encoder 70, the encoder 70 outputs Q=1 together with a 10-bit signal "101010101" with CDS=0 corresponding to "00" for Q'=-1 delayed by one symbol from the immediately preceding output Q=-1. As a result, the DSV value at the end of the codeword after NRZI modulation remains at +2. Then, when "FF" is inputted to the encoder 70, the encoder 70 outputs Q=-1 together with a 10-bit signal with CDS=-2 corresponding to "FF" for Q'=1. As a result, the DSV at the end of the codeword after NRZI modulation becomes zero. In this manner, for each 8-bit dataword inputted to the encoder 70, a codeword to be output is selected from the table for either Q'=-1 or Q'=1 corresponding to the dataword on the basis of the immediately preceding table selection signal output. Consequently, the value of the DSV at the end of each codeword after NRZI modulation is limited to 0, +2 or -2 . This indicates that the DSV divergence is suppressed, thus realizing DC-free data conversion that has no DC component.
In the data conversion method of the prior art, as described above, codewords with CDS=0 are mapped in one-to-one corresponding relationship to datawords; the problem here is that when DSV&gt;0 or DSV&lt;0, if a dataword corresponding one-to-one to a codeword with CDS=0 is input, the DSV cannot be converged fast enough.
In the prior art 8-10 modulation method, since Tmin is as small as 0.8 data clock cycle (T), the intersymbol interference is large, and since Tw.times.Tmin shows a relatively small value of 0.64, there remains some problem in realizing high-density recording. On the other hand, in the prior art 8-14 modulation method, since Tmax/Tmin shows a relatively large value of 3.5, the prior art has had the problem that the peak shift due to intersymbol interference tends to occur and the signal overwrite characteristics tend to degrade.